1. Field of the Invention
The present invention relates to a coherent optical communication apparatus and method.
2. Description of the Related Art
A coherent optical communication system is a system, in which a continuous wave signal from a laser diode is modulated with an electrical signal carrying information to be transmitted using amplitude modulation, frequency modulation and/or phase modulation at the transmitting side. The modulated optical signal from the transmitting side and a local oscillator signal are coupled for optical detection at the receiving side. In case the frequency of the local oscillator signal is the same as the optical carrier of the modulated optical signal, it is called homodyne detection, or in some cases intradyne detection. In case the frequency of the local oscillator signal is different from the one of the carrier component, it is called heterodyne detection.
In case of heterodyne detection, a detector outputs an electrical IF (Intermediate Frequency) signal. The IF is equal to the frequency difference between the carrier of the modulated optical signal and the local oscillator signal. In case of homodyne/intradyne detection, the detector directly outputs an electrical baseband signal. In coherent optical communication systems, it is required to synchronize the frequency of the local oscillator signal with the modulated optical signal to be received for a correct demodulation process at the receiving side. For this purpose, Christoph Wree et al, “Measured Noise Performance for Heterodyne Detection of 10-Gb/s OOK and DPSK”, IEEE PHOTONICS TECHNOLOGY LETTERS Vol. 19, No. 1, pp. 15-17, January 2007, discloses a configuration that has an AFC (Automatic Frequency Control) loop for controlling the frequency of the local oscillator signal.
However, it requires a high-performance AFC loop to realize the frequency control of the local oscillator signal, and therefore it makes the optical communication apparatus expensive. To solve the above mentioned problem, Satoshi Tsukamoto et al., “Coherent Demodulation of Optical Multilevel Phase-Shift-Keying Signals Using Homodyne Detection and Digital Signal Processing”, IEEE PHOTONICS TECHNOLOGY LETTERS Vol. 18, No. 10, pp. 1131-1133, May 2006, discloses a demodulation configuration without using a high-performance AFC loop or an optical PLL (Phase Lock Loop). The above document relates to a coherent optical communication system using M-ary DPSK (Differential Phase Shift Keying) modulation, where M equals to power of two. According to the above document, each symbol of an electrical signal obtained by the coherent optical detection is raised to the M-th power, the phase error is estimated using an average of several successive symbols, and then the signal is demodulated using the estimated phase error.
However, the average value needs to be divided by M to compensate the calculation of the M-th power. Therefore, if the phase error is not within a range of −π/M to +π/M, it is not possible to compensate it correctly. Further it can be applied only to the optical communication system, which uses M-ary DPSK modulation.